URBEM FECISTI - ENGINEERING & ARCHITECTURE

The articles of the series Urbem Fecisti are dedicated to the way Rome has shaped the modern world, to its modernity. It's a fact that Roman Empire is still living inside each of us in the western world and not only.

Pantheon, a masterpiece of engineering and architecture

Hadrian was one of the greatest emperors of Rome, for sure the one that, more than anyone else, patronized arts, literature and philosophy. By many, he is considered to have be the most wise emperor.
Nowadays we can still admire its legacy in many works, starting from the Hadrian's Wall, one of the most important landmark in Britain, to Hadrina's Villa in Tivoli, near Rome, to its mausoleum, now known as Castel Sant'Angelo in Rome.
However the most impressive among the buildings that were erected under Hadrian's rule is for sure the Pantheon in Rome. He rebuilt the temple dedciated to all Gods, originally built by Marcus Vipsanius Agrippa during the empire of Augustus, that was destroyed by a fire in the year 80 AD. What remains of the original buildg is the inscription in the front part and the to massive bronze doors, each one having the weight of 7 tons.

Before going on , I wish to ask you a question: whinch of the two walls in the picture below, both built essentailly using the same technique, brick binded by concrete, is the older and what differences do you observe? The walls are located few hudreds meters away one from the other, so weather conditions are the same. You can place your answwer in the comments. 

The Pantheon is probably the most influencial building ever built. It has given inspiration to architects, and not only to them, across the centuries. In Rome, Naples, Berlin, Istanbul, Washington D.C. and in many other cities of the world,  one can find copies of the Panteon or of parts of it.

And what to say about its floor? Don't you see some similarity with the painings by Piet Mondrian?

However, I am not an architect or a painter, so you might look to the video below for more information. I am an engineer and the engineering aspects are absolutely stunning.

First of all, the Pantheon is a building that has been in continuous use and open to the public since its construction, 1900 years ago, without requiring reconstructions or major maintenances.
Its dome is the largest ever built in unreinforced concrete. Even St. Petet's dome, despite the appereance, is smaller.

• The rings that are visible from outside are used to convert the horizontal thrusts into vertical ones.
• The dome was made with a single casting of concrete, but using heavier and more resistant filler materials at its base and lighter in the upper part.
• The thickness of the dome is progressively drecreasive from the base to the top.
• The oculus on the top of the dome has not only an architectonical function, but also a structural one. The top of a dome is the point where a collapse might occur. The opening is reinforced with voussoirs to distribute the thrusts.
• The cells that are visible internally do not have only a decorative function. The empty spaces allow to make the dome lighter while, at the same time, the reticular structure contributes to a correct distribution of the thrusts.
• There is no direct analysis of the tensile stresses but computerized simulations show that they are less than 10 % of the allowable maximum for the type of concrete that has been used. 

A question might rise about the oculus: why has it been left open without covering it with a glass? The reason is the diffraction of the light that is operated by a glass. The sun rays entering from the oculus are forming a disk that, according to the hour and the day, are illuminating different points of the internal part of the building. And that has been a precise calculation: at noon, on April 21, the birthday of Rome, sun rays illuminate exactly the entrance door. At that moment the emperor was entering in the temple to start the celebrations. 

Anyhow the real secret of the Pantheon, as well as the secret of many other Roman buildings, is the cement. Ancient buildings have survived over the centuries. If today we often see only ruins, thta has been due in some cases to the lack of a minimum maintenance, in other cases to natural disasters and, in most of the cases, to the human action. But deterioration of the building is somenthing that occurred very rarely.

Cement was already known earlier but it was not used for structural purposes but rather as a glue to keep together stones. 

The Romans greatly developed the technology to prepare the cement and the concrete so that it became possible to use it directly for structural purposes like in the dome of the Pantheon or to strongly bind stones and bricks.
Unfortunately the formulation of the cement that was used by the Romas has been lost in the Middle Age. Only very recently the development of sophisticated analytical equipment is allowing to understand something more (click here for more information).
It's a matter of facts that the Roman buildings are still standing  after 2000 years of continuous use while building that have been made with modern cement, even with the best one like the Portland type, suffers by degradation after few decades.
In addition to that, the Roman cement required much less energy for its preparation when compared to the one that we use today. Also it allowed very quick contruction works. The construction of the Pantheon required only 7 years and Haghia Sophia in Constantinopole (now Istanbul), still using the same type of materials, only 5 years. On the opposite, the construction of famous stone-made domes like St. Peter in Rome, Santa Maria del Fiore in Florence, St. Paul in London, the Pantheon in Paris, required from a minimum of 14 to a maximum of 50 years.

Vincenzo

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